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Hazard

Hazard. Hazard. تاخيرها ممکن است باعث پالس هاي ناخواسته شوند Glitch هازارد: مداري که احتمال ايجاد glitch در آن هست، هازارد دارد. دو نوع: استاتيک ديناميک. مثال. Initially: X=Y=Z=1. Static Hazard. Definition:

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Hazard

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  1. Hazard

  2. Hazard • تاخيرها ممکن است باعث پالس هاي ناخواسته شوند • Glitch • هازارد: • مداري که احتمال ايجاد glitch در آن هست، هازارد دارد. • دو نوع: • استاتيک • ديناميک

  3. مثال Initially: X=Y=Z=1

  4. Static Hazard • Definition: • A static-1 hazard is the possibility of a 0 glitch when we expect the output to remain at a nice steady 1 based on a static analysis • Formal Definition: • A static-1 hazard is a pair of input combinations that: (a) differ in only one input variable and (b) both give a 1 output; such that it is possible for a momentary 0 output to occur during a transition in the differing input variable.

  5. مثال • Even though “static” analysis predicts that the output is 1 for both input combinations X,Y,Z = 111 and X,Y,Z = 110, F goes to 0 for one unit time.

  6. Static-0 Hazard • A properly designed two-level sum-of-products (AND-OR) circuit has no static-0 hazards. • A static-0 hazard is just the dual of a static-1 hazard •  an OR-AND circuitthat is the dual of the example circuit would have a static-0 hazard.

  7. مثال • تحليل کنيد:

  8. تشخيص هازارد • A Karnaugh map can be used to detect static hazards in a two-level SOP or POS circuit.

  9. رفع هازارد

  10. مثال

  11. Dynamic Hazard • Dynamic hazard: • the possibility of an output changing more than once as the result of a single input transition. • Multiple output transitions can occur if there are multiple paths with different delays from the changing input to the changing output.

  12. مثال • سه مسير از X به F • W,X,Y,Z = 0,0,0,1F=1 • فرض: X يک شود. • فرض: بقية گيت ها سريعند. • اول تغييرات مشکي F=0 • بعد آبي غير ايتاليک • بعد آبي ايتاليک

  13. طراحي مدار بدون هازارد • Techniques for findinghazards inarbitrary circuits, are rather difficult to use. •  when you require a hazard-free design, it’s best to use a circuit structurethat is easy to analyze. • In particular, two-level AND-OR circuit has no static-0 or dynamic hazards. • Static-1 hazards may exist in such a circuit but they can be found and eliminated using K-map.

  14. نکته • Most Hazards are not hazardous: • A well-designed, synchronous digital system is structured so that hazard analysis is not needed for most of its circuits. • In a synchronous system, all of the inputs to a combinational circuit are changed at a particular time, and the outputs are not “looked at” until they have had time to settle to a steady-state value. • Hazard analysis and elimination are typically needed only in the design of asynchronoussequential circuits, • Asynchronous circuits are not the mainstream but if you want to design them, an understanding of hazards will be absolutely essential for a reliable result.

  15. Pulse-Shaping Circuit D B A C F A' ·A = 0 3 gate delays D remains high for three gate delays after A changes from low to high F is not always 0!

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